Thinly laminated formations can hold significant hydrocarbon reservoirs. Often such formations are anisotropic and exhibit the classical “Low Resistivity Pay” (see Boyd et al., “The lowdown on low-resistivity pay,” Oilfield Review, Autumn 1995, pages 4-18 and Worthington, “Recognition and evaluation of low resistivity pay,” Petroleum Geoscience, 2000, Volume 6, pages 77-92, incorporated by reference herein in their entireties). The cause of this very large anisotropy of resistivity (i.e., larger than 3) is likely due to the presence of water bearing thin beds (shale layers for example) and oil bearing sand layers (see Klein, “Saturation effects on electrical anisotropy,” The Log Analyst, January-February 1996, pages 47-49 and Klein et al., “The petrophysics of electrically anisotropic reservoirs,” The Log Analyst, May-June 1997, incorporated by reference herein in their entireties).
Conventional techniques to measure water saturation in thin bed laminates (i.e. in the presence of anisotropic resistivity) are generally in two categories:
Models Using Horizontal Resistivity Only:
By taking into account the effect of shale upon resistivity one can derived a shaly-sand equation to compute Sw. Numerous equations have been proposed such as Waxman-Smits, Indonesian etc. At least 19 equations have been published. A good review can be found in: Worthington, P. F., The Evolution of Shaly-Sand Concepts in Reservoir Evaluation, The Log Analyst, 26, pp. 2340, 1985 (incorporated by reference herein in its entirety).
Models Using Vertical and Horizontal Resistivity:
Following the work done by Klein (1996) and Klein et al. (1997), Shray et al. (see Shray et al., “Evaluation of laminated formations using nuclear magnetic resonance and resistivity anisotropy measurements,” SPE 72370, 2001, incorporated by reference herein in its entirety) proposed an algorithm for water saturation computation in thin-bedded formations, where the inputs are the horizontal resistivity, the vertical resistivity, the fine grained volumetric fraction (from NMR) and the water resistivity. In this approach, the thin-bed is seen as a bi-modal system composed of coarse grain layers and fine grain layers. If the horizontal and vertical resistivity and the amount of fine grain material are known, then the resistivity of the coarse grain and fine grain layer may be calculated. Then the amount of water in each layer may be determined using Archie's law. Consequently, the total amount of water may be determined.
These conventional methods are multi-step inversions requiring knowledge of formation water resistivity and porosity. These inversions are complicated and difficult to transform into real time field answer products.
Accordingly, it is an object of the present invention to provide a method to determine water saturation in thin bed formations from anisotropic resistivity measurements.
Another object of the present invention is to provide a method that directly computes water saturation from the vertical and horizontal resistivity without requiring the determination of porosity of the sand and shale layers, the water resistivity.